1. Field of the Invention
This invention is concerned with sheets coated with a water-insoluble, non-conductive copolymer of methacrylic acid or maleic acid. More particularly it relates to electrographic sheets and to processes for producing and using such materials. This invention especially relates to latex compositions useful in the preparation of sheets having a dielectric coating.
2. Description of the Prior Art
There are two general methods for the use of electrostatic charges in printing processes. One method is to apply a relatively uniform surface charge to a material capable of holding a charge, then selectively discharge the material to form a pattern of charged area. The other method is to selectively apply a surface charge in a desired pattern to a material capable of holding a charge. In either method, the desired pattern is developed by applying a material, generally colored, which adheres to the charged portion.
There is some confusion in the terminology of the prior art of printing processes which make use of electrostatic charges. For clarity, certain terms are used throughout this specification with the following meanings. Consistent with the definitions of Carlson, an early contributor to the art, the term "electrophotographic printing" is applied to printing processes utilizing photoconductivity. This process operates through selective discharging of a charged area by photoconductivity. The term "electrophotographic sheet" or "photoconductive sheet" is applied to thin materials in any desired shape or configuration having a layer of photoconductive material capable of accepting an electrostatic charge and particularly adapted for printing through use of photoconductivity. The term "electrographic printing" is applied to printing processes utilizing electrostatic charges on dielectric surfaces and does not involve photoconductivity. The term "electrographic sheet" or "dielectric sheet" is applied to thin materials in any desired shape or configuration having a dielectric layer particularly adapted for electrographic printing. In electrographic printing, selectively charging the dielectric sheet is most commonly practiced although selective discharging of a charged sheet can be carried out. The term "electrostatic sheets" includes both dielectric sheets and photoconductive sheets.
A dielectric coated sheet for electrographic printing usually is prepared by coating one surface of a substrate with an organic solution of a resin and then volatilizing the solvent to form a non-conductive, continuous film on that surface of the substrate. The substrate usually is electrically conductive or the opposite side of the sheet is coated with a conductive material. When such a sheet is subjected to a source of electrostatic charge in certain predetermined areas, the sheet accepts a charge in those areas -- which must be retained for a sufficient time for a visible image to be developed on the sheet. The image is developed by depositing a toner on the surface of the coated sheet. The toner is attracted to, and adheres to, the charged portions of the sheet which, if desired, may be subjected to elevated temperatures to fuse the toner to the sheet.
The use of organic solvents in the preparation of dielectric sheets results in certain disadvantages such as toxicity and fire hazard as well as the cost of solvents. The resulting requirements for solvent recovery equipment and ventilating means also cause increases in processing costs. Some attempts have been made, therefore, to replace the volatile organic solvent with water. A problem with such systems has been that, although they operate satisfactorily at low relative humidity, they fail to hold a charge to the desired extent at high relative humidity such as above about 75 percent. Some procedures and compositions to alleviate these problems are described in U.S. Pat. No. 3,261,709 issued to Joseph Shulman and U.S. Pat. No. 3,558,544 issued to A. J. Cole et al.